Quality control methods for gas diffusion media

ABSTRACT

The present invention discloses various methods in which a quality of a diffusion media for use in a fuel cell assembly can be qualitatively evaluated. The material constant, an internal contact angle, a ratio of high and low energy pores, and an external contact angle can be calculated for the diffusion media and compared to a predetermined standard to evaluate the quality of the diffusion media.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/342,898, filed on Jan. 15, 2003, the disclosure of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to fuel cells that use a gas diffusionmedia and, more particularly, to quality control methods for gasdiffusion media.

BACKGROUND OF THE INVENTION

Fuel cells have been used as a power source in many applications andhave been proposed for use in electrical vehicular power plants toreplace internal combustion engines. In proton exchange membrane (PEM)type fuel cells, hydrogen is supplied to the anode of the fuel cell andoxygen is supplied as the oxidant to the cathode. PEM fuel cells includea membrane electrode assembly (MEA) comprising a thin, protontransmissive, non-electrically conductive solid polymer electrolytemembrane having the anode on one of its faces and the cathode on theopposite face. The MEA is sandwiched between a pair of electricallyconductive elements which (1) serve as current collectors for the anodeand cathode, and (2) contain appropriate channels and/or openingstherein for distributing the fuel cell's gaseous reactants over thesurfaces of the respective anode and cathode catalysts. A plurality ofindividual cells are commonly bundled together to form a PEM fuel cellstack. The term fuel cell is typically used to refer to either a singlecell or a plurality of cells (stack) depending on the context. A groupof cells within the stack is referred to as a cluster. Typicalarrangements of multiple cells in a stack are described in U.S. Pat. No.5,763,113, assigned to General Motors Corporation.

In PEM fuel cells hydrogen (H₂) is the anode reactant (i.e., fuel) andoxygen is the cathode reactant (i.e., oxidant). The oxygen can be eithera pure form (O₂), or air (a mixture of O₂ and N₂). The solid polymerelectrolytes are typically made from ion exchange resins such asperfluoronated sulfonic acid. The anode/cathode typically comprisesfinely divided catalytic particles, which are often supported on carbonparticles, and admixed with a proton conductive resin. The catalyticparticles are typically costly precious metal particles. These membraneelectrode assemblies, which comprise the catalyzed electrodes, arerelatively expensive to manufacture and require certain controlledconditions in order to prevent degradation thereof.

Efficient operation of a fuel cell depends on the ability to effectivelydisperse reactant gases at catalytic sites of the electrode wherereaction occurs. In addition, effective removal of reaction products isrequired so as to not inhibit flow of fresh reactants to the catalyticsites. Therefore, it is desirable to improve the mobility of reactantand product species to and from the MEA where reaction occurs.

To improve the mobility of reactant and product species to and from theMEA where reactions occur, a diffusion structure which enhances masstransport to and from an electrode in a MEA of a fuel cell is used. Thediffusion structure cooperates and interacts with an electrode at amajor surface of the electrode opposite the membrane electrolyte of thecell, therefore, electrical and heat conductivity are required. Thediffusion structure is typically a composite diffusion medium whichfacilitates the supply of reactant gas to the electrode. The diffusionstructure also facilitates movement of water and the products of thereactions. The typical diffusion structure includes a characteristicbulk layer having two or more portions, such as a PTFE coating and/or amicroporous layer, each with various properties, includinghydrophobicity and surface energy. The bulk layer is also usable aloneto function as a diffusion structure. However, it is preferably combinedwithin an absorption layer and a desorption layer on respective sides ofthe bulk layer to form a preferred diffusion structure. The diffusionstructure, either the bulk layer alone or combined with other layers, ishereinafter referred to as a diffusion media. See for example U.S. Pat.No. 6,350,539 issued to the assignee of the present application. Thediffusion media is positioned between the MEA and the cathode or anodeflow channels of an individual fuel cell.

The quality of a diffusion media is hard to control due to there onlybeing a few tests indicating the performance of a diffusion media.During the manufacturing of a diffusion media, there can be severalsteps. A first step can include a hydrophobization step, such asteflonization of the bulk layer (applying PTFE to the bulk layer) orcoating the bulk layer with other low surface energy substance(s) and asecond step can include coating the hydrophobized bulk layer with amicroporous substrate. To date, the PTFE content is routinely checked byweight and/or by fluorine mapping using a scanning electronic microscope(SEM). The coating can also be visually checked. The weight check is notvery significant due to averaging the weight gain for the whole sheet(bulk layer). That is, the amount of PTFE at any given location is notknown; rather, the total amount of PTFE on the bulk layer is determinedand used to calculate an average PTFE content on a per unit basis. Thus,the weight check can not identify specific areas of the diffusion mediathat have an undesirable PTFE content nor provide a quantitative measureindicative of performance and surface energy. Fluorine mapping is notalways desirable because it is expensive and time consuming. The visualtest can identify contrasting dark and light spots in the diffusionmedia coating which are indicative of problems during fabrication. Thevisual test, however, is a qualitative check that is only useful inspotting large area defects. For controlling the microporous layer, theweight and the gas flow through the diffusion media can be checked forquality control. Additionally, the thickness of the microporous layercan be used for quality assurance purposes. These methods, however, donot appear to adequately relate to the properties effecting theperformance of the diffusion media in a fuel cell. Thus, an improvedmethod for quality control is needed.

SUMMARY OF THE INVENTION

The present invention discloses novel techniques to determine quality ofa diffusion media. The quality determination is based on propertiesaffecting performance of the diffusion media in a fuel cell. Moreparticularly, quality is based upon the property of surface energy, alsorelated to wettability and liquid handling. The novel techniques measureliquid handling properties of diffusion media directly and morepreferably, quantitatively.

In one aspect of the present invention, a method of determining aquality of a diffusion media for use in a fuel cell is disclosed. Themethod includes the steps of: (1) determining a material constant for adiffusion media based on imbibing of a complete wetting liquid by saiddiffusion media; and (2) comparing the material constant to apredetermined standard.

In another aspect of the present invention a different method ofdetermining a quality of a diffusion media for use in a fuel cell isdisclosed. The method includes the steps of: (1) determining an internalcontact angle of a diffusion media based on imbibing of a partialwetting liquid by said diffusion media; and (2) comparing the internalcontact angle with a predetermined standard.

In a different aspect of the present invention, yet another method ofdetermining a quality of a diffusion media for use in a fuel cell isdisclosed. The method includes the steps of: (1) determining a ratio ofa volume high energy pores in at least a portion of a diffusion mediaand a volume of low energy pores in the same portion of the diffusionmedia; and (2) comparing the ratio with a predetermined standard.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a schematic view of an unassembled fuel cell assembly whichincludes a membrane electrode assembly and diffusion media;

FIG. 2 is a pictorial illustration of a cross-section of a membraneelectrode assembly;

FIG. 3 is an exploded cross-sectional view of a multilayered cathodediffusion media;

FIG. 4 is a flow chart showing the steps of the various methods that canbe utilized to test and assess the quality of a diffusion mediaaccording to the principles of the present invention;

FIG. 5 is a simplified graphical representation of an arrangement thatcan be used to measure a weight increase over time or as a function ofposition of a diffusion media in contact with a liquid;

FIG. 6 is a graph of experimental data showing a mass increase squaredover time for a diffusion media comprised of a base material only whendipped in n-heptane and methanol;

FIG. 7 is a graph of experimental data of a mass increase squared overtime of a PTFE coated diffusion media with a microporous layer whendipped in n-heptane and 2-propanol; and

FIG. 8 is a graph of experimental data of a weight change of a PTFEcoated diffusion media with a microporous layer as a function ofposition when dipped in a non-wetting liquid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

The present invention provides a variety of methods for testing aquality of a diffusion media that can be used in a fuel cell assembly,such as the proton exchange membrane (PEM) fuel cell assembly 20 shownin FIG. 1. Referring to FIGS. 1 and 2, the fuel cell assembly 20comprises a membrane electrode assembly 22 which comprises an ionomermembrane 24. An anode electrode 26 is on one side of the membrane 24,and a cathode electrode 28 is on the other side. A cathode diffusionmedia 30 is on the side of cathode electrode 28 facing away frommembrane 24. An anode diffusion media 32 is on the side of the anodeelectrode 26 facing away from membrane 24. The anode side furtherincludes a plate 34 which serves as a gas distributor and currentcollector. Plate 34 may be an end plate or a bipolar plate that servesto separate adjacent cells in a fuel cell stack. Optionally, a gasket 36is included between membrane 24 and plate 34. Plate 34 has surfacefeatures in the form of recesses which provide a fuel gas flow channel38 and un-recessed features referred to as lands 40. The cathode side issimilarly configured with a plate 42 having surface features in the formof recesses which provide an oxidant gas flow channel 43 and un-recessedfeatures referred to as lands 44, and is electrically conductive.Optionally, a gasket 45 is included between membrane 24 and plate 42.

Diffusion media 30 and 32 provide an important function in the operationof the fuel cell assembly 20. Diffusion media 30 and 32 cooperate withelectrodes 26 and 28 and plates 34 and 42 which have flow field channels(grooves) 38 and 43 to promote the transport and removal of water andgases, as well as heat and electrons, to and from fuel cell assembly 20.Referring now to FIG. 3, it can be seen that diffusion media 30 and 32can be comprised of multiple components or layers. While only thecathode diffusion media 30 is shown in FIG. 3, it should be understoodthat anode diffusion media 32 can be comprised of similar or differentcomponents and layers, as needed, to provide desired properties.Diffusion media 30 has a base material 46 on which additional layers orcoatings can be applied to facilitate and enhance the functioning ofdiffusion media 30. For example, base material 46 can be provided with apolytetrafluoroethylene (PTFE) coating 48 to provide desiredcharacteristics and properties for diffusion media 30. Additionally, amicroporous layer 50 can also be applied to one or more of the surfacesof diffusion media 30 to again provide desired characteristics andproperties for diffusion media 30. Specifics about desired properties ofdiffusion media and materials of construction can be found in U.S. Pat.No. 6,350,539 to Wood, III et al., entitled “Composite Gas DistributionStructure for Fuel Cell,” which is incorporated herein by reference.Different ways of applying coatings and/or layers to a diffusion mediaare described in International Publication No. WO 97/50143, entitled“Gas Diffusion Electrode,” and in European Patent Application EP0731520, entitled “Materials for Use in Catalytic Electrode Manufacture”the disclosures of which are incorporated by reference herein.

By the present invention, it has been determined that the surface energyof the diffusion media is indicative of the performance of the diffusionmedia in facilitating the mobility of reactants to and from the MEA.Diffusion media, however, are sensitive to changes in the surfaceenergy. The changes in the surface energy can be caused by a variety ofthings. For example, finger grease from handling the diffusion media orother contaminants can change the surface energy. Additionally,deviation in a process parameter (e.g., temperature, concentration,time, etc.) during manufacturing especially during coating operationscan also change the surface energy of the diffusion media. When thesurface energy of the diffusion media has been changed in a negativeway, operation of a fuel cell using the diffusion media may beunacceptable. For example, unstable operation of the fuel cell under wetconditions can occur due to flooding. Additionally, a decreased lifespanof the fuel cell stack can also be encountered when diffusion mediahaving an unacceptable surface energy are used in a fuel cell.

Referring to FIG. 4, the present invention provides various methods forquantitatively determining whether a diffusion media 30 and 32 isacceptable for use in fuel cell assembly 20. When it is desired to testa diffusion media, as indicated in step 58, four different tests, asindicated in steps 60, 61, 62 and 63, according to the principles of thepresent invention, are available to assess a quality and/or anacceptability of a diffusion media. These methods encompass varioustechniques, described below, that can be employed to quantitatively testthe quality of the base material 46 and the various coatings and/orlayers that are applied to the base material 46. Additionally, thesetechniques can be used to test the diffusion media 30 and 32 at variousstages of manufacture to detect quality problems that may occur duringthe various stages. Additionally, these methods are directly related tothe properties affecting the performance of diffusion media 30 and 32 inthe fuel cell assembly 20. Thus, the methods provide for quality controlby measuring liquid handling properties directly and quantitatively. Theresults are compared to predetermined standards to ascertain whetherdiffusion media 30 and 32 are within tolerances, as indicated in step64, and suitable for use in fuel cell assembly 20, additionalprocessing, and/or additional testing. If the diffusion media 30 and 32are not within tolerances, the diffusion media is rejected, as indicatedin step 66. The predetermined standards can be based on empirical dataor theoretical considerations. The predetermined standards can beabsolute values, ranges, minimums and maximums.

All of the tests 60, 61, 62 and 63 involve a sample 52 of diffusionmedia being placed in contact with a liquid 54. The liquid 54 used canbe either a complete wetting liquid, a partial wetting liquid, or anon-wetting liquid depending upon the specific test and step beingperformed. A complete wetting liquid is a liquid that is adsorbed intoand fills every pore in a solid and results in a contact angle θ for thesolid of θ=0°, a partial wetting liquid is a liquid that is adsorbedinto and fills only a portion of the pores in a solid and results in acontact angle θ for the solid of 0°<θ<90°, while a non-wetting liquid isa liquid that is not adsorbed into any pores in the solid and results ina contact angle θ for the solid of θ≧90°. Whether liquid 54 is acomplete wetting liquid, a partial wetting liquid or a non-wettingliquid will depend on the nature of the liquid and the composition ofthe diffusion media. That is, for a given liquid, whether that liquid isa complete wetting liquid, a partial wetting liquid or a non-wettingliquid will depend upon the composition of the diffusion media. Forexample, methanol may be a complete wetting liquid for a diffusion mediacomprised of only base material 46 and a partial wetting liquid for adiffusion media comprised of base material 46 with a PTFE coating 48while heptane may be a complete wetting liquid for a diffusion mediacomprised of either base material 46 only or base material 46 with aPTFE coating 48. Thus, the specific liquid chosen will vary. Preferably,a complete wetting liquid chosen has a surface tension of less thanabout 22 mN/m. Various liquids have these properties, such as n-hexane,n-heptane, and silicone oil, all of which can be used as a completewetting liquid.

The methods of the present invention that are used to determine aquality of diffusion media 30 and 32 are based on the properties ofdiffusion media 30 and 32 that reflect upon its ability to transportfluids. The methods of the present invention utilize a number ofdifferent techniques to measure different properties of diffusion media30 and 32 to determine whether diffusion media 30 and 32 are acceptable.One tool that is used in these methods is the Washburn adsorptiontechnique which provides the basis for tests 60, 61 and 62. The Washburnadsorption technique is based on adsorption of a partially wettingliquid into the porous diffusion media 30 and 32. By measuring a weightincrease over time an internal contact angle θ inside the porousdiffusion media 30 and 32 can be calculated based on the Washburntheory. The Washburn theory states that when a liquid is brought incontact with a solid surface, the square of the mass of liquid which isadsorbed by capillary action is directly proportional to the adsorptiontime (length of time after the two are brought in contact). It is alsodependent on physical properties of the liquid and solid and isexpressed mathematically for a wetting liquid (either partial orcomplete) in Equation 1 as $\begin{matrix}{t = \frac{2{\eta \cdot m^{2}}}{{C \cdot \delta^{2} \cdot \pi^{2} \cdot \sigma \cdot \cos}\quad\theta}} & {{Equation}\quad 1}\end{matrix}$where t is the time after the liquid is brought in contact with thesolid, m is the mass (or weight) of liquid adsorbed, η is the liquidviscosity, C is the material constant of the solid, ρ is the liquiddensity, σ is the liquid surface tension, and θ is the internal contactangle of the solid surface for the wetting liquid. The material constantC is analogous to porosity, but is somewhat more specific because it isa function of mean pore radius (r) and number of pores in a given samplesize (n). Its theoretical expression appears in Equation 2 asC=r _(i) ⁵ ·n _(i) ²   Equation 2where r_(i) is the mean pore radius and n_(i) is the number of poreswith mean pore radius r_(i). Using Equations 1 and 2, it can quickly bedetermined that wetting time is inversely proportional to mean poreradius and number of pores. Furthermore, the wetting time is moresensitive to the radius of the pores than the actual number of porespresent, indicated by the respective powers of these two parameters.Equation 1 also shows that wetting time increases with increasinginternal contact angle θ, and that as θ approaches 90°, the theoreticaltime for adsorption approaches infinity (cos 90°=0). The internalcontact angle θ and material constant C are determined experimentally bymeasuring a weight increase over time of a sample of diffusion media 30and 32 dipped in a wetting liquid.

To measure the weight increase over time (when using the Washburntechnique) a sample 52 of diffusion media is dipped into a liquid 54 sothat an edge of sample 52 is in complete contact with liquid 54. FIG. 5shows a graphical representation of an arrangement that can be used tomeasure the weight increase over time of sample 52. Sample 52 isattached to a mass detecting instrument, such as a tensiometer, by awire and dipped into the liquid 54. The tensiometer measures the weightincrease of the sample 52 and can be connected to a data collector, suchas a microprocessor 56, which records the change in weight of sample 52as a function of time.

As stated above, tests 60, 61 and 62 are all based on the Washburnadsorption technique. Test 60 comprises testing the material constant Cof the diffusion media which is determined using steps 60 a-60 c, asdescribed below. Test 61 comprises testing the internal contact angle θof the diffusion media which requires that the material constant C bedetermined, as indicated in step 61 a. Step 61 a is accomplished byperforming steps 60 a-60 c to determine the material constant C, asdescribed below. Test 62 comprises testing a ratio of high and lowenergy pores in the diffusion media which does not require that thematerial constant C be determined. Thus, when it is desired to test adiffusion media using either tests 60 or 61, the first step is todetermine the material constant C of the diffusion media, as indicatedin steps 60 a-60 c of test 60 and step 61 a of test 61.

The material constant C can be found by modifying Equation 1 to solvefor C which results in Equation 3. $\begin{matrix}{C = {\frac{m^{2}}{t} \cdot \frac{2 \cdot \eta}{{\delta^{2} \cdot \pi^{2} \cdot \sigma \cdot \cos}\quad\theta}}} & {{Equation}\quad 3}\end{matrix}$To solve for material constant C, sample 52 is dipped into a completelywetting liquid (internal contact angle θ=0°) which causes the term cos θto drop out of Equation 3 (cos 0=1). The sample 52 is dipped in thecomplete wetting liquid so that at least an edge of sample 52 is incomplete contact with the liquid and the weight increase over time ismeasured. Preferably, sample 52 is dipped in the complete wetting liquidin a way that an edge of sample 52 is strictly parallel with the liquidsurface so that the edge is in complete contact with the liquid. A graphof the weight increase squared over time of the sample 52 in a completewetting liquid, in this case n-heptane, is shown in FIG. 6 and indicatedas curve 68. As can be seen, the weight increase in sample 52 approachesa maximum limit which represents a saturated condition wherein thecomplete wetting liquid has saturated the sample 52. As can be seen whenlooking at Equation 3, the term $\frac{m^{2}}{t}$represents a slope of the curve 68. A portion 70, which is substantiallylinear, of curve 68 is used to calculate the slope$\left( \frac{m^{2}}{t} \right)$which is then used to solve for material constant C. Portion 70 of curve68 is chosen to reflect a substantially steady state adsorption ofliquid 54 by sample 52. As can be seen, the portion 70 chosen begins ata time after the initial contact between the sample 52 and liquid 54 (toavoid meniscus effects and non-steady state adsorption) and ends priorto saturation (to avoid non-steady state adsorption) because all theother terms (properties) other than $\frac{m^{2}}{t}$are known, with the calculation of the slope of portion 70, the materialconstant C for sample 52 when placed in a completely wetting liquid canbe determined. The slope of portion 70 can be determined using a varietyof statistical techniques. For example, a least squares fit techniquecan be performed to determine a slope for portion 70. If test 60 isbeing performed, the material constant C is compared to a predeterminedstandard, as indicated in step 60 d, to ascertain if the diffusion media30 and 32 is within tolerances, as indicated in step 64. If the materialconstant C is within tolerances, the diffusion media is acceptable andcan be tested further, used in a fuel cell assembly 20, or furtherprocessed. If the material constant C is not within tolerances, thediffusion media is rejected, as indicated in step 66.

If test 61, testing internal contact angle θ, is being performed, thematerial constant C determined above is used (or determined using steps60 a-60 c if test 60 was not performed). Once the material constant Chas been obtained, an internal contact angle θ for sample 52 can bedetermined using Equation 1. Equation 1 can be rearranged to solve forthe internal contact angle θ and expressed as shown in Equation 4.$\begin{matrix}{\theta = {\cos^{- 1}\left\lbrack {\frac{m^{2}}{t} \cdot \frac{2 \cdot \eta}{\delta^{2} \cdot \pi^{2} \cdot \sigma \cdot C}} \right\rbrack}} & {{Equation}\quad 4}\end{matrix}$To determine the internal contact angle θ, a sample 52 of diffusionmedia of substantially the same geometry and size as the sample used forthe determination of the material constant, is dipped in a partialwetting liquid 54 so that at least an edge of sample 52 is in completecontact with the partial wetting liquid 54, as indicated in step 61 b.The weight increase over time of sample 52 in the partial wetting liquidis measured and recorded, as indicated in step 61 c. A graph of theweight increase squared versus time of sample 52, in this case a sampleof diffusion media comprised solely of base material 46, in the partialwetting liquid, in this case methanol, is shown in FIG. 6 and indicatedas curve 72. As can be seen, the weight increase in sample 52 approachesa maximum limit which represents a saturated condition, for thisparticular partial wetting liquid, wherein all pores that can be filledwith the partial wetting liquid have been filled. The slope$\left( \frac{m^{2}}{t} \right)$of curve 72 appears in Equation 4 along with material constant C andother known components/properties. Again, a portion 74, which issubstantially linear, of curve 72 is used to calculate the slope$\left( \frac{m^{2}}{t} \right)$which is then used to solve for the internal contact angle θ usingEquation 4, as indicated in step 61 d. Portion 74 is chosen based on thesame criteria discussed above with reference to portion 70 of curve 68.Once internal contact angle θ has been obtained it is compared to apredetermined standard, as indicated in step 61 e, to ascertain if thediffusion media from which sample 52 was taken is within tolerances, asindicated in step 64. If the internal contact angle θ is withintolerances, the diffusion media is acceptable and can be tested further,used in a fuel cell assembly 20, or further processed. If the internalcontact angle θ is not within tolerances, the diffusion media isrejected, as indicated in step 66.

EXAMPLE 1 Testing of Base Material of Diffusion Media

A. Testing Material Constant C

A sample of base material 46 was tested to determine its materialconstant C and its internal contact angle θ. A sample of base material46 was dipped in n-heptane (a complete wetting liquid) and the increasein weight squared as a function of time as a result of absorption of thecomplete wetting liquid is shown as curve 68 in FIG. 6. A portion 70 ofcurve 68 is used to determine the material constant C for the basematerial 46. Portion 70 comprises the data collected betweenapproximately 1 to 5 seconds. An equation describing portion 70 wasdeveloped and is shown in FIG. 6. As can be seen based on the R² value,portion 70 is substantially linear. Using Equation 3, the data forn-heptane, and the slope of portion 70 of curve 68, all shown in Table1, the material constant C for base material 46 is calculated asfollows: TABLE 1 Property Property Viscosity [mPas] 0.4105 Surfacetension [mN/m] 20.4 Density [Kg/m³] 0.697 Slope [g²/s] 0.0019 Contactangle 0 (complete wetting liquid)

$C = {{\frac{m^{2}}{t} \cdot \frac{2 \cdot \eta}{{\delta^{2} \cdot \pi^{2} \cdot \sigma \cdot \cos}\quad\theta}} = {{\frac{0.0019\frac{g^{2}}{s}}{1000^{2}\frac{g^{2}}{{Kg}^{2}}} \cdot \frac{{2 \cdot 0.0004105}\frac{Ns}{m^{2}}}{0.697^{2}{\frac{{kg}^{2}}{m^{6}} \cdot \pi^{2} \cdot 0.024}{\frac{N}{m} \cdot {\cos(0)}}}} = {1.59 \times 10^{- 11}m^{5}}}}$The material constant C can then be compared to a predetermined standardto ascertain the quality of base material 46.

B. Testing Internal Contact Angle θ

A sample 52 of the same base material 46 was dipped in methanol, apartial wetting liquid, and a resulting plot of the increase in weightsquared as a function of time is shown as curve 72 in FIG. 6. A portion74 (between approximately 3 and 8 seconds) of curve 72 is used to testthe internal contact angle θ. A curve fit for portion 74 was developedand is shown in FIG. 6. The slope of portion 74 is shown in Table 2along with the properties of methanol and the material constant C of thediffusion media as calculated above. Using the data in Table 2 andEquation 4, the internal contact angle θ of base material 46 when dippedin methanol is calculated as shown below. TABLE 2 Property PropertyViscosity [mPas] 0.591 Surface tension [mN/m] 22.4 Density [Kg/m³]0.7914 Slope [g²/s] 0.0017 Material Constant [m⁵] 1.59 × 10⁻¹¹

$\Theta = {{\cos^{- 1}\left\lbrack {\frac{0.0017\frac{g^{2}}{s}}{1000^{2}\frac{g^{2}}{{Kg}^{2}}} \cdot \frac{{2 \cdot 0.000591}\frac{Ns}{m^{2}}}{0.7914^{2}{\frac{{Kg}^{2}}{m^{6}} \cdot \pi^{2} \cdot 0.024}{\frac{N}{m} \cdot 1.59} \times 10^{- 11}m^{5}}} \right\rbrack} = {{{\cos^{- 1}\lbrack 0.912\rbrack}\quad\text{=>}\quad\Theta} = 24.2^{{^\circ}}}}$The internal contact angle θ can then be compared to a predeterminedstandard to ascertain the quality of base material 46.

Some types of diffusion media may have properties and/orcharacteristics, such as high energy pores and low energy pores, whichadsorb a partial wetting liquid at different rates. The different ratesof adsorption by the high and low energy pores can be seen in a curve ofthe weight increase squared versus time which produces two distinctsubstantially linear portions along the curve instead of a singlesubstantially linear portion as described above. For example, whendiffusion media 30 and 32 comprises base material 46 having a PTFEcoating 48 and a microporous layer 50, diffusion media 30 and 32 willhave both high energy pores and low energy pores. The terms high energypores and low energy pores are relative terms wherein high energy poresrefer to pores that adsorb a specific liquid more quickly than lowenergy pores. Diffusion media exhibiting this behavior can also bequantitatively tested using the methods of the present invention.Specifically, material constant test 60, internal contact angle test 61and a ratio of high and low energy pores test 62, described below, canbe used to quantitatively test the diffusion media. Material constant Cis determined using steps 60 a-60 c described above and does not changedue to this different behavior of the diffusion media. Thus, test 60 isperformed in the same manner discussed above and is not discussedfurther.

The phenomenon of high and low energy pores is shown in FIG. 7, which isa plot of weight increase squared versus time of a sample 52 of adiffusion media, having a PTFE coating 48 and a microporous layer 50which was dipped in a partial wetting liquid, in this case 2-propanol,and indicated as curve 75. As can be seen, curve 75 comprises twodistinct substantially linear portions identified as a first portion 76and a second portion 77. As can be seen, the slope of first portion 76is steeper than the slope of second portion 77. This is due to the factthat first portion 76 represents adsorption of the 2-propanol by boththe high energy and low energy pores while second portion 77 representsfurther adsorption primarily caused by the low energy pores only as thehigh energy pores have substantially reached a saturation point. Thatis, because the high energy pores adsorb the 2-propanol quicker than thelow energy pores, the high energy pores will reach a saturationcondition before the low energy pores reach saturation. As a result, thesecond portion 77 of curve 75 has a smaller slope reflective ofadsorption by the low energy pores and not the high energy pores. Thefirst and second portions 76 and 77 of curve 75 are chosen using similarcriteria discussed above in reference to portion 70 of curve 68.Additionally, first and second portions 76 and 77 can also be chosen soas to have respective ending and starting points that avoid thetransition period where the primary adsorption mode changes fromadsorption by both high and low energy pores to adsorption by low energypores.

When it is desired to test the internal contact angle θ of a diffusionmedia having high and low energy pores, test 61 can be performed bytesting the internal contact angle θ of the low energy pores and/or theinternal contact angle θ of the high energy pores. That is, first andsecond portions 76 and 77 can be used to determine the internal contactangle θ for the low energy pores and the high energy pores.Specifically, the slope of the second portion 77 can be used inconjunction with Equation 4 and the material constant C, determined instep 61 a, to calculate an internal contact angle θ for the low energypores as indicated in step 61 d and described above. The slope of thefirst portion 76 requires further data manipulation before an internalcontact angle θ for the high energy pores can be calculated due to thefact that the first portion 76 is comprised of adsorption of the partialwetting liquid by both high and low energy pores. The portion of theslope of the first portion 76 attributable to adsorption only by thehigh energy pores can be determined by subtracting the slope of secondportion 77 from the slope of first portion 76. The slope of the highenergy pores can then be used in conjunction with Equation 4 andmaterial constant C, determined in step 61 a, to calculate the internalcontact angle θ for the high energy pores, as indicated in step 61 d anddescribed above.

EXAMPLE 2 Testing a PTFE Coated Diffusion Media with a Microporous Layer

A. Testing Material Constant

A sample 52 of a PTFE coated diffusion media with a microporous layerwas dipped in n-heptane, a complete wetting liquid, and an increase inweight over time was measured. The results were plotted in FIG. 7 as aweight increase squared over time and indicated as curve 78. Curve 78has a portion 79, which is substantially linear, that is used todetermine material constant C. As was discussed above, an equationdescribing portion 79 of curve 78 is developed and is shown in FIG. 7.The slope of portion 79 of curve 78 and the properties of n-heptane areshown below in Table 2. Using the data in Table 2 and Equation 3,material constant C for the PTFE coated diffusion media with amicroporous layer is calculated as follows. TABLE 2 Property PropertyViscosity [mPas] 0.4105 Surface tension [mN/m] 20.4 Density [Kg/m³]0.697 Slope [g²/s] 0.0008 Contact angle 0 (complete wetting liquid)Applied to equation 3:$C = {{\frac{m^{2}}{t} \cdot \frac{2{\eta \cdot}}{{\delta^{2} \cdot \pi^{2} \cdot \sigma \cdot \cos}\quad\Theta}} = {{\frac{0.0008\frac{g^{2}}{s}}{1000^{2}\frac{g^{2}}{{Kg}^{2}}} \cdot \frac{{2 \cdot 0.0004105}\frac{N}{m^{2}}s}{0.697^{2}{\frac{{kg}^{2}}{m^{6}} \cdot \pi^{2} \cdot 0.0204}{\frac{N}{m} \cdot {\cos(0)}}}} = {0.67 \times 10^{- 11}m^{5}}}}$Material constant C can then be compared to a predetermined standard toascertain if the PTFE coated diffusion media with a microporous layer iswithin tolerances and suitable for further processing, testing and/oruse in a fuel cell assembly 20.

B. Testing Internal Contact Angle

Once material constant C has been determined, an internal contact angleθ for the PTFE coated diffusion media with a microporous layer can bedetermined by conducting test 61. A different sample 52 of the same PTFEcoated diffusion media with a microporous layer was dipped in2-propanol, a partial wetting liquid, and an increase in weight overtime was measured. The results are plotted in FIG. 7 as curve 75, whichis a plot of the increase in weight squared over time. As was mentionedabove, curve 75 has a first and second portion 76 and 77 that havedistinct and different slopes. Equations describing first and secondportions 76 and 77 were developed and are shown in FIG. 7. Thedifference between the slopes of portions 76 and 77, as was describedabove, is due to the fact that first portion 76 represents the2-propanol being adsorbed by both the high and low energy pores whilesecond portion 77 is reflective of the 2-propanol being adsorbedprimarily by the low energy pores due to the high energy pores havingsubstantially obtained a saturated condition. The slope of secondportion 77 of curve 75 can be used to determine the internal contactangle θ for the low energy pores using the data in Table 3 and Equation4 and is calculated below. To determine an internal contact angle θ forthe high volume pores, the contribution to the slope of first portion 76of curve 75 due to the high energy pores adsorbing the 2-propanol needsto be determined. The slope of second portion 77 of curve 75 issubtracted from the slope of first portion 76 of curve 75 to calculatethe slope of first portion 76 attributable to adsorption by the highenergy pores and is shown in Table 3 below. The properties of 2-propanoland the slopes for the high and low energy pores are shown in Table 3.Using the data in Table 3 and Equation 4, the internal contact angle θfor both the low and high energy pores is calculated as follows: TABLE 3Property Property Viscosity [mPas] 2.39 Surface 20.93 tension [mN/m]Material Constant [m⁵] 0.67 × 10⁻¹¹ Slope Low 0.00016 Energy Pores[g²/s] Density [Kg/m³] 0.7809 Slope High 0.00024 Energy Pores [g²/s]

$\begin{matrix}{\Theta_{{Low}\quad{Energy}} = {\cos^{- 1}\left\lbrack {\frac{0.00016\frac{g^{2}}{s}}{1000^{2}\frac{g^{2}}{{Kg}^{2}}} \cdot} \right.}} \\\left. \frac{{2 \cdot 0.00239}\frac{Ns}{m^{2}}}{0.7809^{2}{\frac{{Kg}^{2}}{m^{6}} \cdot \pi^{2} \cdot 0.02039}{\frac{N}{m} \cdot 0.67} \times 10^{- 11}m^{5}} \right\rbrack \\{= {{{\cos^{- 1}\lbrack 0.92\rbrack}\quad\text{=>}\quad\Theta_{{low}\quad{energy}}} = {23.1{^\circ}}}}\end{matrix}$ $\begin{matrix}{\Theta_{{High}\quad{Energy}} = {\cos^{- 1}\left\lbrack {\frac{0.00024\frac{g^{2}}{s}}{1000^{2}\frac{g^{2}}{{Kg}^{2}}} \cdot} \right.}} \\\left. \frac{{2 \cdot 0.00239}\frac{Ns}{m^{2}}}{0.7809^{2}{\frac{{Kg}^{2}}{m^{6}} \cdot \pi^{2} \cdot 0.02039}{\frac{N}{m} \cdot 0.67} \times 10^{- 11}m^{5}} \right\rbrack \\{= {{{\cos^{- 1}\lbrack 1.09\rbrack}\quad\text{=>}\quad\Theta_{{high}\quad{energy}}} = {undefined}}}\end{matrix}$The internal contact angle θ for the low energy pores can be compared toa predetermined standard to ascertain if the PTFE coated diffusion mediawith a microporous layer is within tolerances and suitable for use in afuel cell assembly 20, for additional processing, and/or additionaltesting. The internal contact angle θ for the high energy pores,however, is undefined. This indicates that the internal contact angle θof the high energy pores cannot be measured with 2-propanol as thepartial wetting liquid. That is, 2-propanol is a complete wetting liquidwith respect to the high energy pores in this particular PTFE coateddiffusion media with a microporous layer. Thus, if it is desired todetermine an internal contact angle θ for the high energy pores, apartial wetting liquid other than 2-propanol will need to be used.

When the diffusion media contains both high and low energy pores,another test according to the principles of the present invention isavailable to quantitatively ascertain a property of the diffusion media,as indicated in step 62. In the test of step 62, a ratio of high and lowenergy pore volumes is determined and used to quantitatively ascertainthe quality of the diffusion media. Specifically, the total pore volumeof high energy pores and the total pore volume of low energy poreswithin a diffusion media are determined, as indicated in steps 62 d and62 e, and a ratio of the two pore volumes is calculated, as indicated instep 62 f, and compared to a predetermined standard, as indicated instep 62 g, to ascertain whether the diffusion media has acceptableproperties, as indicated in step 64. To determine the pore volume ofhigh energy pores and low energy pores a sample 52 of the diffusionmedia is first dipped in a partial wetting liquid, as indicated in step62 a and described above with reference to step 61 b, and a weightincrease over time of sample 52 is measured, as indicated in step 62 band described above with reference to step 61 c, and a total pore volumeof diffusion media 30 and 32 is calculated, as indicated in step 62 c.The total pore volume is determined by measuring the total weightincrease when sample 52 is saturated by the partial wetting liquid,subtracting the weight increase caused by initial effects between sample52 and the liquid and then dividing by the density of the partialwetting liquid, as shown in Equation 5 $\begin{matrix}{V_{{total}{(l)}} = \frac{m_{SL} - m_{IL}}{\delta_{L}}} & {{Equation}\quad 5}\end{matrix}$where V_(total(L)) is the total pore volume of sample 52 when dipped inliquid L, m_(SL) is the weight increase of sample 52 due to saturationof sample 52 with liquid L, m_(IL) is the weight increase of sample 52due to initial effects between sample 52 and the liquid L and not due toadsorption, and δ_(L) is the density of liquid L.

After determining the total pore volume of sample 52 for the liquid 54,the volume of high energy pores is calculated, as indicated in step 62d. The volume of high energy pores is calculated by determining theweight increase in sample 52 due to adsorption of liquid 54 by the highenergy pores and dividing by the density of liquid 54. To determine theweight increase in sample 52 caused by adsorption of liquid 54 by thehigh energy pores, first portion 76 of curve 75 is used. First portion76 represents a weight increase squared in sample 52 due to adsorptionof liquid 54 by both the high and low energy pores and due to initialeffects between sample 52 and liquid 54. Therefore, the contributionattributable to the low energy pores and attributable to the initialeffects needs to be subtracted to ascertain the weight increase due toadsorption only by the high energy pores. To accomplish this, the weightincrease, m_(IL), due to initial effects between sample 52 and liquid 54and the weight increase due to adsorption by low energy pores issubtracted from the weight increase due to adsorption by both the highand low energy pores. The weight increase due to initial effects betweensample 52 and liquid 54 is the square root of m₀ in FIG. 7. M₀ is theinitial weight increase squared of sample 52 when initially put incontact with liquid 54. The weight increase due to adsorption by boththe high and low energy pores is the square root of m₁ in FIG. 7. M₁ isthe weight increase squared of sample 52 substantially at the transitionfrom adsorption by both the high and low energy pores to adsorptionprimarily by the low energy pores. The weight increase caused byadsorption by low energy pores is ascertained from the slope of secondportion 77 of curve 75 and multiplying it by t₁, the time to reach m₁.The resulting mass increase due to adsorption by the high energy poresis then divided by the density of the liquid to determine the volume ofhigh energy pores and is represented by Equation 6 $\begin{matrix}{V_{{total}{(l)}} = \frac{m_{SL} - m_{L}}{\delta_{L}}} & {{Equation}\quad 5}\end{matrix}$where V_(highenergy (L)) is the pore volume of high energy pores, m₁ isthe squared weight increase due to both high and low energy poresadsorbing liquid L, m₀ is the squared weight increase due to initialeffects between sample 52 and liquid L, a_(low) is the slope of thesquared weight increase over time due to adsorption by low energy pores(slope of second portion 77 of curve 75), t₁ is the time to reach m₁,and δ_(L) is the density of liquid L.

Once the pore volume of the high energy pores has been calculated, thevolume of low energy pores is calculated, as indicated in step 62 e, bysubtracting the volume of high energy pores from the total pore volumeas shown in Equation 7V _(lowenergy(L)) =V _(total(L)) −V _(highenergy(L))   Equation 7where V_(lowenergy(L)) is the volume of low energy pores for liquid L,V_(total (L)) is the total pore volume for liquid L, andV_(highenergy(L)) is the volume of high energy pores for liquid L. Theratio of the volume of high energy pores to the volume of low energypores is then calculated, as indicated in step 62 f, using Equation 8$\begin{matrix}{V_{{highenergy}{(L)}} = \frac{\sqrt{m_{1} - m_{0} - \left( {a_{low} \cdot t_{1}} \right)}}{\delta_{L}}} & {{Equation}\quad 6}\end{matrix}$where ρ_((L)) is the ratio of the volume of high energy pores to thevolume of low energy pores. This ratio can then be compared to apredetermined standard, as indicated in step 62 g, and whether thediffusion media is within tolerances is ascertained, as indicated instep 64. If the ratio is within tolerances, the diffusion media isacceptable and can be tested further, used in a fuel cell assembly 20,or further processed. If the ratio is not within tolerances, thediffusion media is rejected, as indicated in step 66. While the ratioρ_((L)) described above is a ratio of the pore volumes of high energypores to low energy pores, it should be understood that a ratio of thepore volumes of low energy pores to high energy pores can also be usedand compared to an appropriate predetermined standard to ascertain aquality of the diffusion media.

EXAMPLE 3 Testing a PTFE Coated Diffusion Media with a Microporous Layer

Testing Ratio of High and Low Energy Pores

The same PTFE coated diffusion media with a microporous layer that wastested in Example 2, can also be quantitatively evaluated by testing aratio of high and low energy pores. To test the ratio of high and lowenergy pores, a sample 52 of the PTFE coated diffusion media withmicroporous layer is dipped in a partial-wetting liquid while anincrease in weight over time is measured. These steps were done inExample 2 and resulted in curve 75 shown in FIG. 7. The next step is todetermine the total pore volume. The increase in weight squared whenboth the high and low energy pores are saturated is reflected as m₂ inFIG. 7 while the increase in weight squared at a time when the highenergy pores have just reached a saturated condition is represented asm₁. The increase in weight squared caused by initial effects betweensample 52 and the partial-wetting liquid is represented as m₀ in FIG. 7.The numerical values of m₁, m₂ and m₀ are shown in Table 4 below alongwith the properties for 2-propanol. Using Equation 5 in conjunction withthe data in Table 4, the total pore volume for the PTFE coated diffusionmedia with a microporous layer is calculated as follows: TABLE 4Property Property Density [g/cm³] 0.781 t₁[s] 31 m₁[g²] 0.040804$a_{low}\left\lbrack \frac{g^{2}}{s} \right\rbrack$ 0.00016 m₂ = m_(SL)² [g²] 0.051984 m₀ = m_(IL) ² [g²] 0.025

$\begin{matrix}{\eta_{(L)} = \frac{V_{{high}\quad{{energy}{(L)}}}}{V_{{low}\quad{{energy}{(L)}}}}} & {{Equation}\quad 8}\end{matrix}$Now that the total pore volume has been determined, the volume of highenergy pores can be determined. Using Equation 6 and the data in Table4, the volume of high energy pores is calculated as follows:$V_{total} = {\frac{m_{SL} - m_{L}}{\delta} = {\frac{\sqrt{m_{2} - m_{0}}}{\delta} = {\frac{\sqrt{{0.051984\quad g^{2}} - {0.025\quad g^{2}}}}{0.781\frac{g}{{cm}^{3}}} = {0.210\quad{cm}^{3}}}}}$The volume of the low energy pores is now calculated using Equation 7 asfollows:V _(low energy) =V _(total) −V _(high energy)=0.210 cm³−0.133 cm³=0.077cm³Finally, the ratio of the volume of high energy pores to the volume oflow energy pores is calculated using Equation 8 as follows:$\eta = {\frac{V_{{high}\quad{energy}}}{V_{{low}\quad{energy}}} = {\frac{0.133\quad{cm}^{3}}{0.077\quad{cm}^{3}} = 1.73}}$The ratio ρ can then be compared to a predetermined standard toascertain if the PTFE coated diffusion media with a microporous layer iswithin tolerances and suitable for use in a fuel cell assembly 20, forfurther processing, or for further testing.

In another aspect of the present invention, a different method ofquantitatively determining a quality of diffusion media 30 and 32 isused. This method tests an external contact angle θ of the diffusionmedia, as indicated in step 63. The external contact angle θ iscalculated, as indicated in steps 63 d and 63 f, and compared to anappropriate predetermined standard, as indicated in steps 63 e and 63 g,and whether the diffusion media is within tolerances is ascertained, asindicated in step 64. The method of testing the external contact angle θis based on the Wilhelmy adsorption technique which determines anexternal contact angle θ between a non-wetting liquid and a solid bymeasuring a weight change caused by dipping sample 52 in a non-wettingliquid. This technique is fast and inexpensive to run and measuresliquid interaction at the surface of the diffusion media, which is theprimary mechanism for flooding of a catalyst layer.

If it is desired to test the external contact angle θ, as indicated instep 63, a portion of a sample 52 of diffusion media is dipped in anon-wetting liquid 54, as indicated in step 63 a. While sample 52 isbeing placed in and removed from the non-wetting liquid, a weight changein sample 52 is measured and recorded as a function of the depth orposition of sample 52 in the non-wetting liquid, as indicated in step 63b. The arrangement shown graphically in FIG. 5, adapted to measure aweight change as a function of position, can be used to dip sample 52into and out of the non-wetting liquid 54 while recording the change inweight of sample 52.

A graph of a weight change as a function of position of sample 52 ofdiffusion media, in this case a diffusion media comprised of basematerial 46 having a PTFE coating 48 and a microporous layer 50, dippedin a non-wetting liquid, in this case water, is shown in FIG. 8. As canbe seen, the graph has two distinct curves that correspond,respectively, to advancing sample 52 into the non-wetting liquid,indicated as advancing curve 80, and removing or receding sample 52 fromthe non-wetting liquid, indicated as receding curve 82. Advancing curve80 has an advancing non-linear first portion 84 and an advancingsubstantially linear second portion 86. Advancing first portion 84corresponds to a meniscus forming between sample 52 and the non-wettingliquid as sample 52 is advanced or immersed into the non-wetting liquid.Similarly, receding curve 82 has a receding non-linear first portion 88and a receding substantially linear second portion 90. Receding firstportion 88 corresponds to a meniscus forming between sample 52 and thenon-wetting liquid when sample 52 is receding or removed from thenon-wetting liquid. Equations describing the second portions 86 and 90are determined using statistical techniques, such as those discussedabove in reference to portion 70 of curve 68. The first portions 84 and88 that correspond to meniscus formation are not used in determining anexternal contact angle θ for sample 52. Rather, second portions 86 and90 can be used to determine, respectively, an advancing external contactangle θ_(a) and a receding external contact angle θ_(r) for sample 52based on the Wilhelmy technique and the equations describing thecharacteristics of the second portions 86 and 90, as indicated in step63 c.

Specifically, the external contact angle θ of sample 52 when dipped in anon-wetting liquid is determined using the Wilhelmy equation which isshown in Equation 9 $\begin{matrix}{\theta = {\cos^{- 1}\left\lbrack \frac{m_{\sigma} \cdot g}{L_{WL} \cdot \sigma} \right\rbrack}} & {{Equation}\quad 9}\end{matrix}$where θ is the external contact angle, m_(σ) is the weight change causedby the solid/liquid interaction between sample 52 and the non-wettingliquid, L_(WL) is the wetted length (2·(d+w)) where d is the thicknessof sample 52 and w is the width of sample 52, σ is the surface tensionof the non-wetting liquid, and g is the gravity constant. The advancingand receding weight changes (m_(σ)) caused by the interaction betweensample 52 and the non-wetting liquid are determined by setting theposition to zero (x=0) and solving the equations that describe thesecond portions of the respective advancing and receding curves 80 and82. In other words, the non-varying portion of the equations thatdescribe second portions 86 and 90 are used as the respective weightchange (m_(σ)) in calculating the advancing and receding externalcontact angles θ. Thus, an advancing external contact angle θ_(adv)and/or a receding external contact angle θ_(rec), as indicated in steps63 d and 63 f, can be calculated for diffusion media 30 and 32 based ontesting sample 52 in a non-wetting liquid. The resulting externalcontact angles θ are then compared to respective predeterminedstandards, as indicated in steps 63 e and 63 g. Based on the comparisonsto the appropriate predetermined standards, whether the diffusion mediais within tolerances is then determined, as indicated in step 64. If theexternal contact angle θ is within tolerances, the diffusion media isacceptable and can be tested further, used in a fuel cell assembly 20,or further processed. If the external contact angle θ is not withintolerances, the diffusion media is rejected, as indicated in step 66.

EXAMPLE 4 Testing a PTFE Coated Diffusion Media with a Microporous Layer

Testing External Contact Angle

A sample 52 of a PTFE coated diffusion media with a microporous layerwas dipped in water, a non-wetting liquid, while the change in weight asa function of position of the sample was measured. The result ofmeasuring the change in weight as a function of position is shown inFIG. 8 and is represented by advancing curve 80 and receding curve 82.As was discussed above, the second portions 86 and 90 of the respectiveadvancing and receding curves 80 and 82 are used to determine theexternal contact angle θ. An equation describing the advancing secondportion 86 of advancing curve 80 was developed and is shown in FIG. 8.An equation describing the receding second portion 90 of receding curve82 was also developed and is shown in FIG. 8. The known properties ofthe sample (i.e., wetted length, thickness, and width) along with thesurface tension of the water and the gravity constant are known andshown in Table 5 along with the weight change m_(σ) caused by theinteraction between the water and the sample. Using the data in Table 5in conjunction with Equation 9, the advancing internal contact angleθ_(adv) and receding internal contact angle θ_(rec) are calculated asfollows: TABLE 5 Property Advancing Receding m_(σ) −0.5846 −0.084 gWetting Length 82.66 mm Surface Tension 72.8 mN/m Gravity constant 9.81m/s²

${{Advancing}\text{:}\Theta_{adv}} = {{\cos^{- 1}\left\lbrack \frac{m_{\sigma} \cdot g}{L \cdot \sigma} \right\rbrack} = {{\cos^{- 1}\left\lbrack \frac{{- 0.5846}\quad{g \cdot 9.81}\frac{m}{s^{2}}}{82.66 \times 10^{- 3}\quad{m \cdot 72.8}\frac{gm}{s^{2}m}} \right\rbrack} = {{{\cos^{- 1}\left\lbrack {- 0.956} \right\rbrack}\quad\text{=>}\quad\Theta_{adv}} = {162{^\circ}}}}}$${{Receding}\text{:}\Theta_{rec}} = {{\cos^{- 1}\left\lbrack \frac{m_{\sigma} \cdot g}{L \cdot \sigma} \right\rbrack} = {{\cos^{- 1}\left\lbrack \frac{{- 0.084}\quad{g \cdot 9.81}\frac{m}{s^{2}}}{82.66 \times 10^{- 3}\quad{m \cdot 72.8}\frac{gm}{s^{2}m}} \right\rbrack} = {{{\cos^{- 1}\left\lbrack {- 0.137} \right\rbrack}\quad\text{=>}\quad\Theta_{adv}} = {97.8{^\circ}}}}}$The advancing external contact angle θ_(adv) and/or the recedingexternal contact angle θ_(rec) can then be compared to an appropriatepredetermined standard to ascertain if the PTFE coated diffusion mediawith microporous layer is within tolerances and suitable for use in afuel cell assembly 20, further processing, and/or further testing.

In addition to providing a quantitative evaluation of the quality of adiffusion media, the testing methods of the present invention facilitatethe manufacture of a diffusion media having various coatings and/orlayers. That is, the quantitative measure of the properties of thediffusion media can be used to refine further processing steps. Forexample, the material constant and/or the contact angle can be used toprovide a required length of time for base material 46 to be in adispersion containing PTFE particles to apply a PTFE coating of aspecified concentration on base material 46. Additionally, the materialconstant and/or contact angles can be used to aid in the application ofa microporous coating. For example, the material constant and/or contactangles can be used to determine a required composition of a solvent usedto apply the microporous layer 50. Thus, the methods of the presentinvention can be used to aid in the processing and manufacture of adiffusion media.

The above described various methods of the present invention enabledifferent properties of the diffusion media to be quantitativelyevaluated along with allowing diffusion media of various compositions tobe quantitatively evaluated. The various testing method according to thepresent invention include: (1) testing the material constant; (2)testing an internal contact angle; (3) testing a ratio of high and lowenergy pores; and (4) testing an external contact angle (receding and/oradvancing). Additionally, various examples have been provided to furtherexplain and clarify the different testing methods of the presentinvention and the applicability of these methods to diffusion mediahaving various coatings and/or layers. It should be understood that theexamples shown are for purposes of illustration only and should not beconstrued to limit the scope or applicability of the methods of thepresent invention to diffusion media having coatings and/or layersdifferent from those in the examples.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A method of determining a quality of a diffusion media for use in afuel cell, the method comprising the steps of: (a) determining aninternal contact angle of a diffusion media based on imbibing of apartial wetting liquid by said diffusion media; and (b) comparing saidinternal contact angle with a predetermined standard.
 2. The method ofclaim 1, wherein step (a) further comprises determining a relationshipbetween mass increase and time duration of imbibing.
 3. The method ofclaim 2, wherein step (a) further comprises deriving said internalcontact angle based on said relationship.
 4. The method of claim 2,wherein said time duration is a time period prior to saturation of saiddiffusion media by said liquid.
 5. The method of claim 2, wherein saidrelationship exhibits a distinct change in slope between a first portionof said relationship characteristic of high and low energy poresimbibing said fluid and a second portion of said relationshipcharacteristic of low energy pores imbibing said fluid and step (a)further comprises determining an internal contact angle for high energypores based on said first portion.
 6. The method of claim 2, whereinsaid relationship exhibits a distinct change in slope between a firstportion of said relationship characteristic of high and low energy poresimbibing said fluid and a second portion of said relationshipcharacteristic of low energy pores imbibing said fluid and step (a)further comprises determining an internal contact angle for low energypores based on said second portion.
 7. The method of claim 1, whereinsaid partial wetting liquid is isopropanol.
 8. The method of claim 1,wherein said predetermined standard is a range of acceptable internalcontact angles.
 9. The method of claim 1, wherein said predeterminedstandard is based upon empirical data.
 10. The method of claim 1,wherein said partial wetting liquid is methanol.
 11. The method of claim1, further comprising accepting said diffusion media based upon saidcomparison.
 12. The method of claim 1, wherein (a) includes determininga material constant for said diffusion media based upon imbibing of awetting liquid by said diffusion media.
 13. A method of testing adiffusion media for use in a fuel cell, the method comprising: (a)determining a relationship between a mass increase and time duration ofimbibing of a liquid by a diffusion media; (b) determining an internalcontact angle of said diffusion media based on said relationship; (c)comparing said internal contact angle with an empirically basedpredetermined standard; and (d) ascertaining an acceptability of saiddiffusion media based upon said comparison.
 14. The method of claim 13,wherein said liquid is a partial wetting liquid for said diffusionmedia.
 15. The method of claim 13, wherein (b) includes using asubstantially linear portion of said relationship to determine saidinternal contact angle.
 16. The method of claim 13, wherein prior tosaturation said relationship exhibits a distinct change in slope betweena first portion of said relationship characteristic of a faster rate ofimbibing said liquid and a second portion of said relationshipcharacteristic of slower rate of imbibing said liquid and (b) furthercomprises determining an internal contact angle based on said firstportion.
 17. The method of claim 16, wherein (b) comprises determiningan internal contact angle for high energy pores based on said firstportion.
 18. The method of claim 13, wherein prior to saturation saidrelationship exhibits a distinct change in slope between a first portionof said relationship characteristic of a faster rate of imbibing saidliquid and a second portion of said relationship characteristic ofslower rate of imbibing said liquid and (b) further comprisesdetermining an internal contact angle based on said second portion. 19.The method of claim 18, wherein (b) comprises determining an internalcontact angle for low energy pores based on said second portion.
 20. Amethod of testing a diffusion media for use in a fuel cell, the methodcomprising: (a) ascertaining a quality of a diffusion media based upon acomparison of an internal contact angle of said diffusion media to apredetermined standard; (b) accepting said diffusion media for at leastone of use in a fuel cell and application of a coating to said diffusionmedia based upon said ascertained quality; and (c) rejecting saiddiffusion media based upon said ascertained quality if said diffusionmedia quality is unacceptable.